The state of charge of electrochemical energy stores such as lithium-ion batteries, for example, is a fundamental state variable required for indicating the state of charge and for controlling the charging and/or discharging of the store. For reliable operation of an energy storage device, the state of charge has to be determined sufficiently accurately. In practice, however, precisely the determination of the state of charge, particularly in the case of batteries having a flat characteristic curve, such as batteries based on e.g. lithium iron phosphate cells, poses a particular difficulty. A flat characteristic curve means that for a large portion of the states of charge the battery voltage is not dependent on the state of charge or is only very slightly dependent thereon. Therefore, if the state of charge of the battery is intended to be determined by a measurement of the voltage dropped across the battery, the determined state of charge is highly prone to errors on account of this low dependence. During operation, inaccurate state of charge values have the effect that either it is not possible to utilize the entire storage capacity of the battery or even disturbing emergency shutdowns of a system operated with the energy storage device are carried out by the battery management system if voltage limits of the battery are reached.
States of charge have been determined hitherto by the combination of two mutually complementary methods. In the case where currents not equal to zero flow, the state of charge determination is realized by means of charge counters. In this case, the current with which the energy storage device is charged and/or discharged is measured and integrated over time. In combination with a well-defined initial value, the present state of charge is in each case calculated by this means. Inaccuracies in the current measurement, such as e.g. an offset, are integrated over time in this case and can lead to significant deviations from the actual state of charge.
In order to correct this deviation, therefore, the state of charge is additionally determined by means of the quiescent voltage of the energy storage device. However, even in the case of energy storage devices having steep characteristic curves, the relationship between battery voltage and state of charge is provided only if the battery is quiescent, that is to say that no current flows into or out of the battery, over a relatively long period of time, for example 15 minutes. With current not equal to zero, by contrast, the voltage deviates from the quiescent voltage upward or downward depending on the current direction. In this case, this deviation is dependent on numerous factors, in particular the retention profile and temperature. The described correction of the state of charge can therefore be carried out only in periods of time in which the energy storage device is neither charged nor discharged. Since these periods of time are temporally spaced, these corrections generally lead to jumps in the determined state of charge.
The corrections described prove moreover to be significantly more difficult in the case of batteries having a flat characteristic curve. In the case of such stores, the dependence of the quiescent voltage on the state of charge in a large range is too low to be able to carry out a reliable determination of the state of charge. A quiescent voltage correction, as described above, is therefore not possible in the case of these batteries. The state of charge calculated by the charge counter therefore becomes more and more inaccurate over the course of time, as described above, for which reason the state of charge has to be calibrated from time to time. This is possible, for example, by the battery being fully charged and the charge counter being set to 100%. During this process, however, the battery typically cannot be used since charging the battery to 100% is carried out according to a defined procedure. The process may therefore be performed when the store need not be ready for use. If such a calibration is not possible over a relatively long period of time, it is possible that the operation of the battery must be interrupted for the purpose of calibration.